Conventionally, the adapter for the MPO type optical connector is provided with a plug insertion port 100 having a key groove K in an inner portion into which an optical connector plug (not shown) is inserted, in both ends of a horizontally long rectangular casing shaped housing H having a flange F at the center of both right and left side surfaces, and is provided with an arm-shaped locking piece 101 for retaining optical reference planes of the respective optical connector plugs inserted into both the plug insertion ports 100 at a fitting position so as to bring into line with each other in bilaterally symmetrical positions in the inner portion of the plug insertion ports 100, as shown in FIG. 11 and FIGS. 12A to 12E.
At this time, in the case that in a state in which the optical connector plug is not inserted into the one plug insertion port 100, an optical component associated with the optical connector plug connected to the other plug insertion port 100 is in an operating state, a harmful laser beam (for example, 5 mW or more) is conducted to the one plug insertion port 100, so that there has been a risk that the light may harm a worker's body seriously, particularly harm a worker's eye by a direct view of the light from the optical cable, when the worker fits the optical fiber connector to the plug insertion port 100. Further, there has been a risk that a leading end portion of the inserted optical cable (a ferrule and the optical fiber) may give a problem due to attachment of dust.
In order to avoid the risk mentioned above, there has been conventionally an optical coupling part with a shutter in which a clamshell type shutter structured such that two doors are horizontally arranged is provided, the doors rotating inside when an optical connector plug is inserted into a plug insertion port of an optical coupling part main body (an adapter main body), and these two doors of the shutter is fixed in such a manner that convex portions provided in both upper and lower ends of two doors are engaged with concave portions formed in the plug insertion port side of the optical coupling part main body by putting a mask member provided with a lock arm which can be locked to a locking groove in the optical coupling part main body (the adapter main body) side over the plug insertion port, as shown in Patent Literature 1.